Field of the Technology
The present disclosure relates to high strength alpha-beta titanium alloys.
Description of the Background of the Technology
Titanium alloys typically exhibit a high strength-to-weight ratio, are corrosion resistant, and are resistant to creep at moderately high temperatures. For these reasons, titanium alloys are used in aerospace, aeronautic, defense, marine, and automotive applications including, for example, landing gear members, engine frames, ballistic armor, hulls, and mechanical fasteners.
Reducing the weight of an aircraft or other motorized vehicle results in fuel savings. Thus, for example, there is a strong drive in the aerospace industry to reduce aircraft weight. Titanium and titanium alloys are attractive materials for achieving weight reduction in aircraft applications because of their high strength-to-weight ratios. Most titanium alloy parts used in aerospace applications are made from Ti-6Al-4V alloy (ASTM Grade 5; UNS R56400; AMS 4928, AMS 4911), which is an alpha-beta titanium alloy.
Ti-6Al-4V alloy is one of the most common titanium-based manufactured materials, estimated to account for over 50% of the total titanium-based materials market. Ti-6Al-4V alloy is used in a number of applications that benefit from the alloy's advantageous combination of light weight, corrosion resistance, and high strength at low to moderate temperatures. For example, Ti-6Al-4V alloy is used to produce aircraft engine components, aircraft structural components, fasteners, high-performance automotive components, components for medical devices, sports equipment, components for marine applications, and components for chemical processing equipment.
Ductility is a property of any given metallic material (i.e., metals and metal alloys). Cold-formability of a metallic material is based somewhat on the near room temperature ductility and ability for a material to deform without cracking. High-strength alpha-beta titanium alloys, such as, for example, Ti-6Al-4V alloy, typically have low cold-formability at or near room temperature. This limits their acceptance of low-temperature processing, such as cold rolling, because these alloys are susceptible to cracking and breakage when worked at low temperatures. Therefore, due to their limited cold formability at or near room temperature, alpha-beta titanium alloys typically are processed by techniques involving extensive hot working.
Titanium alloys that exhibit room temperature ductility generally also exhibit relatively low strength. A consequence of this is that high-strength alloys are typically more costly and have reduced gage control due to grinding tolerances. This problem stems from the deformation of the hexagonal close packed (HCP) crystal structure in these higher-strength beta alloys at temperatures below several hundred degrees Celsius.
The HCP crystal structure is common to many engineering materials, including magnesium, titanium, zirconium, and cobalt alloys. The HCP crystal structure has an ABABAB stacking sequence, whereas other metallic alloys, like stainless steel, brass, nickel, and aluminum alloys, typically have a face centered cubic (FCC) crystal structures with ABCABCABC stacking sequences. As a result of this difference in stacking sequence, HCP metals and alloys have a significantly reduced number of mathematically possible independent slip systems relative to FCC materials. A number of the independent slip systems in HCP metals and alloys require significantly higher stresses to activate, and these “high resistance” deformation modes are activated in only extremely rare instances. This effect is temperature sensitive, such that below temperatures of several hundred degrees Celsius, titanium alloys have significantly lower malleability.
In combination with the slip systems present in HCP materials, a number of twinning systems are possible in unalloyed HCP metals. The combination of the slip systems and the twinning systems in titanium enables sufficient independent modes of deformation so that “commercially pure” (CP) titanium can be cold worked at temperatures in the vicinity of room temperature (i.e., in an approximate temperature range of −148° F. (−100° C.) to 392° F. (+200° C.)).
Alloying effects in titanium and other HCP metals and alloys tend to increase the asymmetry, or difficulty, of “high resistance” slip modes, as well as suppress twinning systems from activation. A result is the macroscopic loss of cold-processing capability in alloys such as Ti-6Al-4V alloy and Ti-6Al-2-Sn-4Zr-2Mo-0.1Si alloy. Ti-6Al-4V and Ti-6Al-2-Sn-4Zr-2Mo-0.1S alloys exhibit relatively high strength due to their high concentration of alpha phase and high level of alloying elements. In particular, aluminum is known to increase the strength of titanium alloys, at both room and elevated temperatures. However, aluminum also is known to adversely affect room temperature processing capability.
In general, alloys exhibiting cold deformation capability can be manufactured more efficiently, in terms of both energy consumption and the amount of scrap generated during processing. Thus, in general, it is advantageous to formulate an alloy that can be processed at relatively low temperatures.
Some known titanium alloys have delivered increased room-temperature processing capability by including large concentrations of beta phase stabilizing alloying additions. Examples of such alloys include Beta C titanium alloy (Ti-3Al-8V-6Cr-4Mo-4Zr; UNS R58649), which is commercially available in one form as ATI® 38644™ beta titanium alloy from Allegheny Technologies Incorporated, Pittsburgh, Pa. USA. This alloy, and similarly formulated alloys, provides advantageous cold-processing capability by decreasing and or eliminating alpha phase from the microstructure. Typically, these alloys can precipitate alpha phase during low-temperature aging treatments.
Despite their advantageous cold processing capability, beta titanium alloys, in general, have two disadvantages: expensive alloy additions and poor elevated-temperature creep strength. The poor elevated-temperature creep strength is a result of the significant concentration of beta phase these alloys exhibit at elevated temperatures such as, for example, 500° C. Beta phase does not resist creep well due to its body centered cubic structure, which provides for a large number of deformation mechanisms. Machining beta titanium alloys also is known to be difficult due to the alloys' relatively low elastic modulus, which allows more significant spring-back. As a result of these shortcomings, the use of beta titanium alloys has been limited.
Lower cost titanium products would be possible if existing titanium alloys were more resistant to cracking during cold processing. Since alpha-beta titanium alloys represent the majority of all alloyed titanium produced, cost could be further reduced by volumes of scale if this type of alloy were maintained. Therefore, interesting alloys to examine are high-strength, cold-deformable alpha-beta titanium alloys. Several alloys within this alloy class have been developed recently. For example, in the past 15 years Ti-4Al-2.5V alloy (UNS R54250), Ti-4.5Al-3V-2Mo-2Fe alloy, Ti-5Al-4V-0.7Mo-0.5Fe alloy, and Ti-3Al-5Mo-5V-3Cr-0.4Fe alloy have been developed. Many of these alloys feature expensive alloying additions, such as V and/or Mo.
Ti-6Al-4V alpha-beta titanium alloy is the standard titanium alloy used in the aerospace industry, and it represents a large fraction of all alloyed titanium in terms of tonnage. The alloy is known in the aerospace industry as not being cold workable at room temperatures. Lower oxygen content grades of Ti-6Al-4V alloy, designated as Ti-6Al-4V ELI (“extra low interstitials”) alloys (UNS 56401), generally exhibit improved room temperature ductility, toughness, and formability compared with higher oxygen grades. However, the strength of Ti-6Al-4V alloy is significantly lowered as oxygen content is reduced. One skilled in the art would consider the addition of oxygen as being deleterious to cold forming capability and advantageous to strength in Ti-6Al-4V alloys.
However, despite having higher oxygen content than standard grade Ti-6Al-4V alloy, Ti-4Al-2.5V-1.5Fe-0.250 alloy (also known as Ti-4Al-2.5V alloy) is known to have superior forming capabilities at or near room temperature compared with Ti-6Al-4V alloy. Ti-4Al-2.5V-1.5Fe-0.250 alloy is commercially available as ATI 425® titanium alloy from Allegheny Technologies Incorporated. The advantageous near room temperature forming capability of ATI 425® alloy is discussed in U.S. Pat. Nos. 8,048,240, 8,597,442, and 8,597,443, and in U.S. Patent Publication No. 2014-0060138 A1, each of which is hereby incorporated by reference herein in its entirety.
Another cold-deformable, high strength alpha-beta titanium alloy is Ti-4.5Al-3V-2Mo-2Fe alloy, also know as SP-700 alloy. Unlike Ti-4Al-2.5V alloy, SP-700 alloy contains higher cost alloying ingredients. Similar to Ti-4Al-2.5V alloy, SP-700 alloy has reduced creep resistance relative to Ti-6Al-4V alloy due to increased beta phase content.
Ti-3Al-5Mo-5V-3Cr alloy also exhibits good room temperature forming capabilities. This alloy, however, includes significant beta phase content at room temperature and, thus, exhibits poor creep resistance. Additionally, it contains a significant level of expensive alloying ingredients, such as molybdenum and chromium.
It is generally understood that cobalt does not substantially affect mechanical strength and ductility of most titanium alloys compared with alternative alloying additions. It has been described that while cobalt addition increases the strength of binary and ternary titanium alloys, cobalt addition also typically reduces ductility more severely than addition of iron, molybdenum, or vanadium (typical alloying additions). It has been demonstrated that while cobalt additions in Ti-6Al-4V alloy can improve strength and ductility, intermetallic precipitates of the Ti3X-type also can form during aging and deleteriously affect other mechanical properties.
It would be advantageous to provide a titanium alloy that includes relatively minor levels of expensive alloying additions, exhibits an advantageous combination of strength and ductility, and does not develop substantial beta phase content.